High-speed semiconductor lasers are core devices of high-speed communication systems. A high-performance transmitter is a device which uses a high-power and low-noise DFB laser as its light source and loads data by using a direct modulation or an external modulation. The external modulation technology may achieve a wide range of modulation frequency (>75 GHz), but there are some shortcomings, such as large size, high cost, high drive voltage and a large insertion loss (6˜7 dB). A directly modulated semiconductor laser is to superimpose a modulation signal on a DC bias current, and it may modulate amplitude of a laser output signal, and achieves an efficient light emission. Besides, it has many unique advantages, such as a small volume, a low power consumption, a high linearity, and easy to use.
The directly modulated laser may cause a decreased relaxation oscillation frequency due to its internal physical effects and an effect of parasitic parameter, which limits an operating rate of a transmission system. In general, a light output end surface of the semiconductor laser is easily to be optically damaged, so a power density near the end surface of the laser determines an output light power and responding dynamic range of the semiconductor laser.
The output light power and the relaxation oscillation frequency of the semiconductor laser are proportional to an injection current. With an increased injection current, a peak of the relaxation oscillation frequency moves toward the high frequency direction. However, increasing injection current tends to cause an irreversible damage to the end surface of the semiconductor laser, or even burn up the semiconductor laser.